R1:bb2b91d28e7e

Add unit tests for file thumbnail generation

Summary: Fixes T12614.

Test Plan:
- Ran tests, saw them pass.
- Changed a thumbnail to 800x800, saw tests detect that the default image was missing.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12614

Differential Revision: https://secure.phabricator.com/D17773